1. Field of the Invention
The present invention relates generally to an optical information recording and reproducing apparatus and more particularly to an optical information recording and reproducing appparatus which can record an information signal on a recording medium and repeatedly reproduce the recorded information signal therefrom.
2. Description of the Prior Art
A previously proposed optical information recording and reproducing apparatus generally causes the following defects in a recording mode. In general, an optical information recording and reproducing apparatus includes an optical recording medium or a recording disc formed by interposing, for example, a metal layer having a low melting point between transparent resinuous or glass layers as in a sandwich shape. During the recording mode of the apparatus the recording disc is irradiated by a converging high intensity beam in response to information signals which causes a partial melting of the melt layer thereof and thereby results in a spirally arranged recording track formed of pits or indicia.
If the converging state of, for example, the laser beam is poor, the pits or indicia which comprise the recording track may not be perfectly formed or may not be formed at all on the recording disc creating a defective and unacceptable recording track.
In particular, if such an apparatus includes an objective lens with a large numerical aperture (NA) to effectively utilize the laser beam, then even a slight defocusing of the lens causes the aforesaid defective and unacceptable recording track.
In an attempt to overcome this defect, and as previously proposed, a beam, such as the laser beam when used in the recording mode, is reflected from the recording disc and thereafter detected by an optical or photodetector whereby the converging condition of the laser beam is controlled by the detected output thereof.
However, inasmuch as the reflected beam is produced by reflecting the laser beam onto indicia, which are created during the melting state, it is impossible to detect the quality of the shape of the indicia before they are completely formed. For this reason, the converging condition of the recording laser beam can not be adequately or positively controlled and therefore makes it quite difficult to achieve an acceptable track recording.
Additionally, it is highly desirable to use as many of the same circuit elements, in the apparatus, for both the recording and reproducing modes as possible in order to reduce the manufacturing cost of the apparatus.